In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into service areas or cell areas, with each service area or cell area being served by radio network node such as an access node e.g. a Wi-Fi access point or a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. The service area or cell area is a geographical area where radio coverage is provided by the access node. The access node operates on radio frequencies to communicate over an air interface with the wireless devices within range of the access node. The access node communicates over a downlink (DL) to the wireless device and the wireless device communicates over an uplink (UL) to the access node.
A Universal Mobile Telecommunications System (UMTS) is a third generation telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High-Speed Packet Access (HSPA) for communication with user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for present and future generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several access nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural access nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, such as 4G and 5G networks. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long-Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a 3GPP radio access technology wherein the access nodes are directly connected to the EPC core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising access nodes connected directly to one or more core networks.
With the emerging 5G technologies, the use of very many transmit- and receive-antenna elements is of great interest as it makes it possible to utilize beamforming, such as transmit-side and receive-side beamforming. Transmit-side beamforming means that the transmitter can amplify the transmitted signals in a selected direction or directions, while suppressing the transmitted signals in other directions. Similarly, on the receive-side, a receiver can amplify signals from a selected direction or directions, while suppressing unwanted signals from other directions.
Beamforming allows the signal to be stronger for an individual connection. On the transmit-side this may be achieved by a concentration of the transmitted power in the desired direction(s), and on the receive-side this may be achieved by an increased receiver sensitivity in the desired direction(s). This beamforming enhances throughput and coverage of the connection. It also allows reducing the interference from unwanted signals, thereby enabling several simultaneous transmissions over multiple individual connections using the same resources in the time-frequency grid, so-called multi-user Multiple Input Multiple Output (MIMO).
Overall requirements for the Next Generation (NG) architecture e.g. TR 23.799 v.0.5.0, and, more specifically the NG Access Technology, e.g. TR 38.913 v.0.3.0 will impact the design of the Active Mode Mobility solutions for the New Radio Access Technology (NR), see RP-160671 New SID Proposal: Study on New Radio Access Technology, DoCoMo, compared to the current mobility solution in LTE. Some of these requirements are the need to support network energy efficiency mechanisms, future-proof-ness and the need to support a very wide range of frequencies e.g. up to 100 GHz.
One of the main differences, with respect to LTE, comes from the fact that propagation in frequencies above the ones allocated to LTE is more challenging so that the massive usage of beamforming becomes an essential component of NR. Despite the link budget gains provided by beamforming solutions, reliability of a system purely relying on beamforming and operating in higher frequencies might be challenging, since the coverage might be more sensitive to both time and space variations. As a consequence of that a Signal to Interference plus Noise Ratio (SINR) of a narrow link can drop much quicker than in the case of LTE, see R2-162762, Active Mode Mobility in NR: SINR drops in higher frequencies, Ericsson.
To support Transmit (Tx)-side beamforming at a radio network node, a number of reference signals may be transmitted from the radio network node, whereby the wireless device can measure signal strength or quality of these reference signals and report the measurement results to the radio network node. The radio network node may then use these measurements to decide which beam(s) to use for the one or more wireless devices.
A combination of periodic and scheduled reference signals may be used for this purpose.
The periodic reference signals, typically called beam reference signals (BRS) or Mobility Reference Signals (MRS), are transmitted repeatedly, in time, in a large number of different directions using as many Tx-beams as deemed necessary to cover a service area of the radio network node. These reference signals may be scheduled or on a need-basis e.g., based on the traffic. As the naming indicates, each BRS represents a unique Tx-beam from that radio network node. This allows a wireless device to measure the BRS when transmitted in different beams, without any special arrangement for that wireless device from the radio network node perspective. The wireless device reports e.g. the received powers for different BRSs, or equivalently different Tx-beams, back to the radio network node.
The scheduled reference signals, called channel-state information reference signals (CSI-RS), are transmitted only when needed for a particular connection. The decision when and how to transmit the CSI-RS is made by the radio network node and the decision is signalled to the involved wireless devices using a so-called measurement grant. When the wireless device receives a measurement grant it measures on a corresponding CSI-RS. The radio network node may choose to transmit CSI-RSs to a wireless device only using beam(s) that are known to be strong for that wireless device, to allow the wireless device to report more detailed information about those beams. Alternatively, the radio network node may choose to transmit CSI-RSs also using beam(s) that are not known to be strong for that wireless device, for instance to enable fast detection of new beam(s) in case the wireless device is moving.
The radio network nodes of a NR network transmit other reference signals as well. For instance, the radio network nodes may transmit so-called demodulation reference signals (DMRS) when transmitting control information or data to a wireless device. Such transmissions are typically made using beam(s) that are known to be strong for that wireless device.
Beamforming introduces a possibility to enhance the signal towards a specific location. This enables better signal to noise ratio towards a specific wireless device.
A specific beamforming towards a specific wireless device is handled per Transmission Time Interval (TTI) where a number of factors and measurements are used to determine how the beamforming should look like. With an increasing number of antenna elements, the number of possible beams that theoretically can be created increases a lot.
Consider a wireless communication system, consisting of radio network nodes also referred to as transmission points (TPs), and wireless devices. The radio network nodes employ beamforming, that is, the radio network nodes transmit their power in a prominent direction to increase the received power at the wireless devices. The radio network node may use beams from a finite set of pre-defined beams. It should also be understood the radio network node can use several of the beams at a same time. The radio network node periodically sends reference signals, such as the BRS or MRS, on each of the possible beams.
The wireless device measures the reference signal received power (RSRP) for each of the reference signals e.g. beam reference signal received power (BRSRP). The wireless device then reports the RSRP values back to the radio network node, which may perform a mobility process such as a handover based on the reported RSRP values.
The Reference Signals (RS) that can be used to support Radio Resource Control (RRC) driven mobility:                For wireless devices in connected active mode non-wireless device specific RS for measurements may be used (the wireless device may not need to be aware whether the RS is wireless device-specific or non-wireless device specific)        The non-wireless device specific RS can be found by the wireless device without much configuration        The non-wireless device specific RS encodes an identity        
For wireless devices in connected mode, intra-cell mobility may be handled by mobility without RRC involvement and there may be cases that do require RRC involvement.
The radio interface of the existing wireless communication systems has been devised to support certain major services with different level of Quality of Service (QoS) requirements. Examples of such major services are voice characterized by low data rate, video streaming characterized by low delay and consistently moderate data rate etc. However, the future 5G wireless communication system will increase the data rate manifold with respect to the existing technology as well as enable lower latencies. This in turn will also pave the way for introducing a wide range of new services and applications in addition to supporting the existing ones. The characteristics of the future services may be very different for example in terms of their desired QoS targets, such as data rate, latency and reliability. Examples of such services are ultra-reliable and low-latency communication (URLLC) such as Vehicle to anything (V2X) and factory automation, ultra-high QoS consumer services (e.g., ultra-high quality audio and/or video conference call), applications requiring high precision (e.g., public safety related applications, medical application etc.), industrial applications characterized by very high reliability (e.g., factory automation, autonomous mining, aviation/drone related applications etc.), remote control applications etc.
Assuming LTE as baseline Radio Resource Management (RRM) measurements supporting RRC based mobility are based on a single reference signal per cell, so-called cell-specific RS (CRS). These reference signals are transmitted all the time i.e. in every LTE subframe of a given cell and across the whole system bandwidth. However, in 5G or New Radio Access Technology (NR), some differences are envisioned for the signals used for RRM measurements to support e.g. RRC based mobility. With the usage of beamforming, there may be multiple reference signals defined per cell, so called Mobility Reference Signals (MRSs) and/or beam-specific reference signals (BRSs), each of them carrying at least a beam ID or similar.
These beams, each assigned with at least one MRS/BRS to support RRM measurements in NR, could be of different characteristics in terms of how data or control information is beam-formed in time, frequency, power and code dimensions. Also, these beams may have different beam shapes e.g., half-power beam-width (HPBW), electrical downtilt (EDT), azimuth, and transmission power (TXP). Furthermore, the beams may be optimized for certain services with corresponding data rate and/or reliability requirements. What is more, these beams could be overlaying. In such a setting, the wireless device may use a beam in a non-optimal manner resulting in a reduced or limited performance of the wireless communication network.